In a wireless communication system, when a mobile station is located in an indoor or outdoor weak electromagnetic field, wireless communication between a base station and the mobile station is sometimes difficult. In order to take measures to deal with this situation, a repeater is often disposed as a wireless relay apparatus amplifying and relaying a downlink signal transmitted from a base station and an uplink signal transmitted from a mobile station. By disposing a repeater, it is possible to expand cell coverage (a service area) at a relatively low cost. To date, the number of transmission antennas and receiving antennas in wireless communications between a base station and a repeater and between a repeater and a mobile station have been one, respectively. That is to say, Single Input Single Output (SISO) configurations have been employed.
In this regard, various proposals have been made for wireless relay. For example, Japanese Laid-open Patent No. 2005-229524 has been proposed for a wireless relay system in which a radio signal from a first radio station is relayed by a wireless relay apparatus to be transmitted to a second radio station. In this related-art wireless relay system, the wireless relay apparatus determines the amount of phase correction of the relay signal based on channel information on a transmission path between the first radio station, the second radio station, and the wireless relay apparatus, and corrects a phase of the relay signal based on the amount of phase correction.
Also, in a wireless communication system, Multiple Input Multiple Output (MIMO) is known as a transmission method for increasing transmission rate using multistream with a limited frequency band. In MIMO, a transmission apparatus and a receiving apparatus are provided with a plurality of antennas, respectively. And transmission symbols (multistream) transmitted from the plurality of antennas of the transmission apparatus are mixed on a propagation path, and are received by the plurality of antennas of the receiving apparatus. It is known that if the number of antennas is assumed to be constant, the smaller the correlation between individual streams, the larger the communication capacity by MIMO becomes.
Incidentally, if it is assumed that a repeater (hereinafter referred to as a “MIMO repeater”) which relays multistream by MIMO, for example a 2×2 MIMO repeater, is provided, a general configuration of an overall wireless communication system is thought to be illustrated in FIG. 1. In FIG. 1, a relay station (RS) as a MIMO repeater generally has a configuration in which repeaters with a related-art SISO configuration are disposed in parallel.
FIG. 1 illustrates an example of wireless relay of a downlink signal from a base station (BS) as a transmission station to a mobile station, or User Equipment (UE), as a receiving station. Here, the relay station as a wireless relay apparatus includes two receiving antennas RA-1 and RA-2, which receive signals from the base station, and two transmission antennas TA-1 and TA-2, which transmit signals to the mobile station. As illustrated in FIG. 1, the signals transmitted from the two transmission antennas of the base station are mixed on a propagation path between the base station and the relay station, and are received by the two receiving antennas RA-1 and RA-2 of the relay station. In the relay station, the received signals by the receiving antennas RA-1 and RA-2 are amplified by power amplifiers AMP-1 and AMP-2, and are transmitted from the transmission antennas TA-1 and TA-2. The signals transmitted from the two transmission antennas TA-1 and TA-2 of the relay station are mixed on a propagation path between the relay station and the mobile station, and are received by two receiving antennas of the mobile station.
For the MIMO repeater, it is demanded that the sum total of transmission power from a plurality of transmission antennas be increased, and a service area for the mobile station be expanded. In order to increase the sum total of transmission power from the plurality of transmission antennas, the average input power (in FIG. 1, the average received power) to the individual power amplifiers connected to the plurality of transmission antennas ought to be made equal. This is because all the power amplifiers may be used at an operating point with high output and high efficiency. If there is a difference in the average input power of the individual power amplifiers, the operating points of the individual power amplifiers are restricted by a power amplifier having the largest average input power.
However, if a MIMO repeater is configured only by disposing repeaters having a related-art SISO configuration as illustrated in FIG. 1, it is thought that level differences arise in the average received power obtained from the individual receiving antennas of the MIMO repeater. The level difference in the average received power is caused by variations of the receiving characteristics of a plurality of receiving systems including the receiving antennas, restrictions of the disposition positions of the individual receiving antennas, etc. It is troublesome to manage the receiving characteristics of a plurality of receiving systems including the receiving antennas, and to adjust the disposition positions of the individual receiving antennas so that level differences will not arise in the average received power obtained from the individual receiving antennas.